The present invention is related to circuits in which a field-effect transistor device controls power transfer from an alternating polarity electrical power supply to a load means, particularly when such field-effect transistor devices are capable of being integrated in monolithic integrated circuits.
Various solid state devices have been used in circuits as the primary means for controlling power transfer from an alternating polarity electrical power supply to whatever kind of load means is of interest for use in the circuit. For instance, planar bipolar power transistors have been used but these are devices which are not bidirectional by nature and which exhibit an inherent, more or less irreducible, minimum power dissipation characteristic even when fully switched on. And to be switched fully on, bipolar power transistors require a substantial amount of base current, i.e., control current, especially for higher collector, or load, currents. Furthermore, they are also subject by nature to thermal runaway.
Perhaps more commonly used for controlling alternating polarity power supplies are thyristors of various kinds such as silicon controlled rectifiers and triacs. Such thyristors are switching devices primarily used in alternating polarity power supply control circuits because of their capability for handling relatively large power dissipations when switched fully on and for withstanding substantial reverse voltages when switched fully off. An advantage of these devices over bipolar power transistors is that they require little electrical power at device control gates whether operating in the off condition or in the on condition.
However, such thyristors also have several disadvantages such as being a latching switch, that is, operating only fully on or fully off. Further, the thyristor device can be switched off by sufficiently reducing the current therethrough, and can be switched on by sharp voltage transients thereacross--both results being obtained without any action taking place at the control terminal of the thyristor device. Hence, the control terminal of the thyristor has relatively little continuous control capability. This same control terminal, in many situations, cannot be electrically isolated simply and inexpensively from the load circuit, and may require large triggering currents to switch on the thyristor device. Finally, a thyristor device cannot be easily provided in a monolithic integrated circuit with other circuit components because of its structure and power dissipation.
Hence, better primary power controlling devices are desired for use in controlling power transfer from alternating polarity electrical power supplies in alternating polarity operated circuits. Particularly useful would be a device which could be easily provided in a monolithic integrated circuit along with other circuit components, at least some of which would also be used in controlling power transfer from the alternating polarity power supply used. This would require that such a device not have too large a resistance if switched fully on, despite substantial current loads, but which would have a structure easily fabricated in such an integrated circuit. Further, the device should have a bidirectional current conduction capability for circuits in which current rectification is not desired.
Field-effect transistor devices can have many of the characteristics just described, including having a very symmetrical bidirectional current conducting capability when on. This is certainly so for metal-oxide-semiconductor field-effect transistor (MOSFET) devices which have the advantage of having the gates therein being very well isolated from the channel regions of the device. This isolation aids in providing a circuit to operate the field-effect transistor device when both the circuit and these devices are formed in a monolithic integrated circuit chip, a difficult arrangement when the integrated circuit is to operate with an alternating polarity power supply. Such circuits must permit the operation of other circuit component devices in the monolithic integrated circuit while also controlling power transfers from the alternating polarity power supply through operating the primary power transfer control field-effect device.
Electronic component device theory shows that field-effect transistors are controlled by controlling the voltage appearing between the gate thereof and that one of the connections to the channel regions therein which is effectively serving as the transistor source. Difficulties arise in those circuits using a field-effect transistor to control power transfers from an alternating polarity power supply because the two connections to the channel region of such a transistor serve alternately as the source rather than one of them serving continually as the source.